In the Fischer Esterification, why is it the acid's carbonyl oxygen getting protonated and not the hydroxyl oxygen? This protonation occurs in Step 1 of the mechanism below.

I read in some textbooks that this is due to the carbonyl oxygen being more neucleophilic and the hydroxyl oxygen. Why is this so?

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  • 2
    $\begingroup$ Draw the resonance structures for both options. $\endgroup$
    – Zhe
    Jun 3 '17 at 12:40
  • $\begingroup$ Same reason why an amide protonates on oxygen instead of nitrogen. $\endgroup$ Jun 3 '17 at 16:57

There are a few reason why the carbonyl oxygen gets protonated -

1) The carbonyl oxygen is sp2 hybridized which is more electronegative than the sp3 oxygen. So, it has more tendency to give electrons since it is more negative.

2) The lone pairs of carbonyl carbon are more available since they are not in conjugation whereas the lone pairs of the other oxygen are not much available since they are in conjugation with carbonyl carbon.

3) When carbonyl oxygen donates its electron the resulting structure is more stabilized (because of conjugation) than the structure formed when the other oxygen atom donates.

So, that's why carbonyl oxygen has far more tendency of donating electrons than the -OH grp.

  • $\begingroup$ Thank you for your answer. For point 1), I was thinking that an sp3 hybridized oxygen atom would have lesser s character than an sp2 hybridized oxygen atom, hence sp3 oxygen's lone pairs would be held less closely to the nucleus and hence be more able to abstract a proton than the sp2 oxygen. Is this a misconception on my part? $\endgroup$ Jun 5 '17 at 3:05
  • 1
    $\begingroup$ If it’s more electronegative it should be less prone to protonation since it is better able to stabilise the negative charge prior to deprotonation. $\endgroup$
    – Jan
    Jun 8 '17 at 0:24
  • $\begingroup$ Also with respect to point (1), both oxygens are sp2. $\endgroup$
    – jerepierre
    Jul 4 '17 at 2:26

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